If she puts my child or children at risk by leaving them in a car unattended for anything except an unforeseeable emergency? Absolutely, I would fire her. And no, I am not particularly moved by the factor of being in a foreign country. Moving here, I presume, was her choice. If I am hiring a servant to be a paid substitute parent to my child, I expect that that person do so responsibly and exercise good judgment. That is why I am paying them. If she wishes to risk her own children's safety, that is her decision. If she risks my children's, I can no longer trust that her judgment is reliable and her decisions are responsible.
It's odd to me that you keep using the term "servant." It seems like a very demeaning word.